1. Field of the Invention
The present invention relates to a charge coupled device (CCD) image sensor, and more particularly to a horizontal charge coupled device (HCCD) having a dummy gate, which causes noise signals to flow from the noise transfer region to ground. The dark current, therefore, flows in the opposite direction of the actual signals, thereby decreasing the effect of the dark current.
2. Discussion of the Related Art
Generally, a CCD image sensor comprises a plurality of photodetectors for generating signal charges in accordance with the intensity of the incident light, a plurality of vertical charge coupled devices (VCCDs) for vertically transferring signal charges from photodetectors and a HCCD for horizontally transferring signal charges from BCCDs.
FIG. 1 shows a structure of a HCCD according to the conventional art and FIGS. 2A and 2D are potential distribution diagrams taken along line II-IIxe2x80x2 of FIG. 1. As shown in FIG. 1, first and second poly gates 1 and 2 are alternatively and successively formed on the BCCD channel 5, which are overlapping each other. The channel stop region 7 is electrically connected to ground (i.e., grounded) to have a higher potential. The potential level of the BCCD channel 5 is changed in accordance with the bias applied to the poly gates.
In FIG. 2A, the symbol xe2x80x9c+++xe2x80x9d marked below the second poly gate 2 stands for the impurity regions which have a lower impurity concentration than the BCCD channel 5 in order to form a staircase potential barrier. In a HCCD of the aforesaid structure, the channel stop region 7 is always grounded so that the potential level is higher than that of the BCCD channel 5, as shown in FIG. 2B. If the clock signals Hxcfx861 and Hxcfx862 are applied to poly gates 1 and 2, the noise charges (exe2x80x2) generated in the channel stop region 7 and p-type well (not shown) with the signal charges (e) transferred from BCCDs are transferred into the BCCD channel 5 as shown in FIGS. 2C and 2D. Therefore, the dark current generated in the sensing amplifier 8 is increased.
It is an object of the present invention to provide a HCCD of a CCD image sensor in which a dummy gate and a dummy drain are respectively formed above and within a BCCD channel to cause the actual signals to flow in the opposite direction of the dark current.
To obtain the above object, there is provided a HCCD of a CCD image sensor which includes a plurality of photodetectors for generating signal charges in accordance with the density of the incident light, a plurality of VCCDs for vertically transferring signal charges from photodetectors comprising a BCCD channel having a signal charge transfer region for horizontally transferring signal charges from VCCDs, and a noise charge transfer region for transferring noise charges which are generated therein in the opposite direction of the signal charge transfer region. The signal charge transfer region and noise charge transfer region have a plurality of impurity regions.
A first plurality of poly gates are formed above the signal charge transfer region of the BCCD channel. The first plurality of poly gates further has a plurality of first poly gates and a plurality of a second poly gates. The first poly gates and second poly gates are alternately and successively arranged. A dummy gate is then formed at the introduction portion of the noise charge transfer region of the BCCD channel. A second plurality of poly gates are formed at the noise charge transfer region of the BCCD channel. The second plurality of poly gates further has a plurality of first poly gates and a plurality of second poly gates which are symmetrically arranged with the first plurality of poly gates and separated from the first plurality of poly gates by the dummy gate. A dummy drain for draining the noise charges from the noise charge transfer region is formed at the end portion of the noise charge transfer region of the BCCD channel.